Relationship Between Changes in Voice Pitch and Loudness

Dept. for Speech, Music and Hearing Quarterly Progress and Status Report

Relationship between changes in voice pitch and loudness

Gramming, P. and Sundberg, J. and Ternstrom,¨ S. and Leanderson, R. and Perkins, W. H.

journal: STL-QPSR volume: 28 number: 1 year: 1987 pages: 039-055

http://www.speech.kth.se/qpsr

STL-QPSR 1/1987

A. RELATIONSHIP BETWEEN CHANGES IN VOICE PITCH AND LOL7llPJESS Patricia Gramming*, Johan Sundberg, Sten Ternstrom, Fblf Leanderson**, and William H. Perkins***

Abstract The change in mean fundamental frequency accompanying changes in loudness of phonation during reading is analyzed in nine professional singers, nine nonsingers and in ten male and ten female patients suffering from vocal fatigue and/or functional dysfunction. The subjects read discursive texts with 19-filtered noise in earphones, and some also at voluntarily varied vocal loudness. Also, the healthy voice subjects phonated as softly and as loudly as possible at various fundamental frequencies throughout their pitch ranges, and the resulting mean phonetograms are compared. The mean fundamental frequency was found to increase by between 0.2 and 0.6 semitones per dB equivalent sound level. No great differences were found between these subject groups, although the singers were found to vary their mean fundamental frequency more than the nonsingers. It is possible to explain the voice pitch changes as the passive results of the changes of subglottal pressure applied in order to vary sound level of phonation.

Introduction Recordings of minimum and maximum phonatory sound level as func- tions of fundamental frequency are called phonetograms. These are frequently used in some voice clinics as a help to describe the voice function. The axes of a phonetogram are related to two important phonatory dimensions, namely voice fundamental frequency and loudness of phonation. These parameters reflect, in turn, basic aspects of the voice source that would be revealing to voice function: the pitch of phonation is determined by the vibration frequency of the vocal folds while the phonatory sound level reflects the maximum amplitude of the dif f eren- tiatecl transglottal airflow (Gauffin & Sundberg, 1980; Fant, 1979; Fant, Liljencrants & Tin, 1985). For these reasons, the phonetogram seems promising also in clinical work, especially as the recording time needed is comparatively short.

* Dept. of Phoniatrics, ENT Clinic, Max> Ceneral EBspital. ** Dept . of Phoniatrics , Karolinska S juld~uset, Stockholm. ***Univ. of Southern California, Ins Anqeles, CAI IJSA. STL-QPSR 1/1987

It is a notorious observation that speakers who raise their loudness of phonation also raise their mean voice fundamental frequency. This suggests the existence of habitual pathways in the phonetogram. Such pathways may help understanding the phonetogram from a practical point of view, and are thus interesting to describe. It seems worth- while to find out how subjects vary their mean fundamental frequency and mean sound pressure level (SPL) when asked to speak at different degrees of vocal loudness. Speakers tend to increase loudness of speech with the ambient ~oise level (Lane & Tranel, 1971). With regard to vocal abuse, clinicians discourage speaking under noisy conditions; this implies that speaking loudly may be detrimental to vocal health. Coricert and opera singers, and stage actors, often use their voices at loudness and pitch levels that would render untrained voices hoarse; yet they do not normally suffer vocal damage. It is interesting to see whether the type of voice use, that speaking in noise induces, leads to a combination of mean SPL and mean fundamental frequency different from that chosen when loudness of phonation is voluntarily raised. The aim of the present investigation was to examine the vocal behavior of different groups of speakers with respect to mean fundamental frequency and mean sound level and to relate these data to the subjects' phonetogram. The investigation combines data gathered for two different pur-- poses. One purpose was to explore singers' and nonsingers' phonatory reactions to various noise and auditory feedback conditions. The other was to describe an aspect of status of voice function in dysphonic patients.

Experiment The effects of voluntarily changed phonatory loudness on level of phonation and voice fundamental frequency was analyzed in two groups of subjects: nine male singers and nine male nonsingers, all with no history of voice disorders. One singer had only vocal training whereas the other eight had performed professionally in opera and concert. Also, the effects of reading in noise was studied in these subjects; the subjects heard a loud noise in earphones and were asked to read a text loud enough to get heard through this noise. In addition, the effects of reading in noise was studied in ten male and ten female patients suffering from dysphonic phonastenia, i.e., a functional voice disorder. All the patients were on the waiting list for treatment at the Phoniatric Department, Malmij General Hospital. In the case of the healthy voices, the recordings were carried out in an anechoic room and the sound was picked up by a Sennlleiser (MD 211) microphone at a constant distance in front of the subject's mouth. For STL-QPSR 1/1987

the patients, a sound treated room (4*3*2.3 m) was used and the microphone was an AKG CMSE CElO electret. Voice fundamental frequency was determined from the signal recorded by an accelerometer, fastened to the subject's neck just below the thyroid cartilage; for the patients this microphone was complemented with an electroglottograph. The subject wore a calibrated headset (TDH 49P) with capsules tightly sealing off the ears from external sound. The sound level of the auditory feedback, obviously critical to the effect of the noise, was adjusted so that a vowel yielding a 90 dB SPL at 1 m distance generated a 105 dB SPL in the headset. This approximates the level at which the subjects would hear their own voices without the headset in an anechoic room. In the case of the patients Sennheiser (MD414) headset was used, which does not seal off the ears. In the headset noise was presented. In the case of the healthy voices, a white noise was used filtered at 2.2 kHz by an LP filter with a roll-off of 18 dB/octave, see Fig. 1. In the case of the patients, unfiltered white noise was used which was presented at 70 dB SPL. The sound picked up by the microphone was recorded on one track of a calibrated tape recorder, and on the second track a signal was recorded for fundamental frequency measurement.

MASKING NOISE

0.1 0.2 0.5 1 2 5 10 FREQUENCY (kHz1 Fig. 1. Ilong-term-average spectrum from a third-octave filterbank of the LP-filtered masking noise.

All subjects read a discursive text material with and without the masking noise. In the noise-free condition, the voice patients were asked to read at a normal loudness only, while the healthy voices read two times (Trial I and 11) for at least 30 sec under each of three loudness conditions : ( 1 ) norma 1 conversational loudness, (2) a lotdness level appropriate for an audience, and (3) the loudest level pssible. Phonetograms were made for the healthy voices only, before reading the text. To provide data for phonetograms, the reconunencld procedure is STL-QPSR 1/1987

to have the subject phonate at certain, given pitches as loudly and as softly as possible (Schutte & Seidner, 1983). In this case, however, we used a slightly different procedure. The singers sang triads, while the nonsingers made pitch glides from highest to lowest pitch or vice versa.

Analysis The signal for fundamental frequency analysis was fed to a fundamental frequency extractor connected to a digital analyzer, displaying the fundamental frequency distributions in terms of a histogram. Also, the mean and the standard deviation of fundamental frequency were shown. Using these means, the fundamental frequency mean and standard deviation were determined for each sample. The program also displays fundamental frequency and sound level as function of time. Such graphs were used for analyzing SPL and fundamental frequency in the pitch glides and triads that the subjects performed during the recording sessions. The resulting data were used for plotting phonetograms. As different voices have differing mean fundamental frequencies, the relevant measure is the change in each subject's mean fundamental frequency induced by the experimental conditions. Therefore, this change was determined for each sample, and expressed in semitones (henceforth st). In the case of the healthy voices, a computer program for long- term-average spectrum analysis was used for determining the equivalent sound level, For the voice patients L was determined by a B&K Leq* eq 2218 sound level meter.

Results The results from the two trials of the healthy voices did not show any significant differences (a variance analysis is published elsewhere, ( Perkins, Ternstrom, Sundberg & Gramming, 1987). Fig. 2 shows the average for the various conditions in the healthy voices. The singers produced higher Leq than the nonsingers. It can be noted that the data point for the unmasked conditions adhere to a straight line. This implies that, on the average, the subjects inter- preted "loud" as situated on a logarithmic sound pressure scale midway between "normal" and "loudest". The increment was 9 and 7 dB for the singers and nonsingers, respectively. Under conditions of masking noise, the nonsingers raised their L while the singers reduced their eq somewhat as compared with the loudest reading. Leq Evidently, the mean fundamental frequency varies considerably between subjects, reflecting morphological and habitual differences. Therefore, Fig. 3 shows, on a 1ogariWic scale, the fundamental fre- STL-QPSR 1/1987 - 43 -

50 I I I I NORMAL LOUD LOUDEST MASKED EXPERIMENTAL CONDITION

Fig. 2. Mean Lq observed for nine singer and nine nonsinger subjects (solid and dashed curves) when reading under various conditions. The bars show +/- one standard deviation.

16 I I I I

h yl - - - w C o 14 - - - 5 E - w J) Y 12 - W U - Z - 10 W / LL - -U, 8- >- - U - W 6- s - W 4 - L - Z a - W 2- - t - -

0 I I I I NORMAL LOUD LOUDEST MASKED EXPERIMENTAL CONDITION

Fig. 3. Mean fundamental frequency recorded for nine singer ad nine nonsinger subjects (solid and dashed curves) when reading under the various conditions. The bars slmw +/- one the standard deviation. STL-QPSR 1/1987

SINGERS It,,,,,- ~I,T, 1L - CLF 1L - SCH 12 - 12 - 10 - 10 - 8 - U - 6 - 6 - L - - 2 - 4 2 - j__: 0 - 0 - -2 - 11111, -2 - 50 60 70 80 IIrIIIt 50 60 70 80

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tIII,I, I1IIII- 14 - JON lL 14 - ROL 12 - 1: 12 - 10 - i9 - 8 - ti- 6 - 6- 4 - 4- 2 - 2- 0 - O 0- -2 -2 .* - 50- 60 70 80 /;50 I 60,/I 70 80

Fig. 4A. Mean fundamental frequency rise above average fundamental frequency in normal reading in Trial I as function of L for various experimental conditions in Trials I and 11. eq Crosses, boxes, circles, and triangles: normal, loud, loudest possible, and masked reading, respectively. STL-QPSR 1/1987

IIilIII 1L - MAR 12 -

10 -

8 - - 6 -

4 - -

-2 - IIIIIII. 50 60 70 80

IIIIJII1

1L - JAN 12 -

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Fig. 4B. STL-QPSR 1/1987

quency relative to a personal reference, viz. the lowest fundamental frequency that the subject was able to produce in the phonetgram recordings. The graph represents the average interval between this lowest frequency and the frequency used in the various conditions for the two groups of healthy subjects. The graph is similar to that shown in Fig. 2. The singers' mean fundamental frequency is higher than that of the nonsingers, and they raise their mean fundamental frequency more with increasing loudness of phonation. Both singers and nonsingers reduce their mean fundamental frequency in the noise condition as compared with the loudest condition. As Leq and the change in mean fundamental frequency apparently are interdependent, it is relevant to find the relationship between these two voice parameters. Fig. 4 shows, for each individual voice, the mean fundamental frequency relative to the subject's average fundamental frequency in normal reading as a function of the for the normal, Leq loud, loudest, and masking noise conditions. It can be seen that there is typically a rather linear relationship between these two voice parameters. The relationship was analyzed by means of linear regression. The constants are listed in Table I. Regarding the slope, there were great intersubject differences; the smallest slope is 0.26 st/d~and the greatest is 0.86 st/d~. According to a Mann-Whitney non-parametric test there was no significant difference between the singers and the nonsingers. In most cases the data pints collected in the masking noise condition lie close to the lines suggesting that the subjects retained their normal relationship between voice pitch and loudness change when they read in noise. It is interesting to find out how these data relate to the phonetograms. Fig. 5 shows some examples of phonetograms for two of the singers and two of the nonsingers. Again, a great variability can be observed. Singer subject SC and nonsinger subject KR both show a large phonation area, while singer subject RO and nonsinger subject TO show narrower such areas. This suggests that the differences between nonsingers and singers do not concern those aspects of voice function that are reflected in a phonetogram. This suggestion is further supprted by the phonetograms averaged over all subjects in each group. The averaging was performed in the following way. Each phonation frequency of each subject was normalized with respect to this subject's total range, expressed in the logarithmic semitone unit (cf., Coleman, Mabis & Hinson, 1977). There was no significant difference between the groups with regard to the size of this range: the mean range of the singers and nonsingers were 26.0 and 23.6 st, SD 2.6 and 3.9, respectively. Then, all Lq values falling in the same decade of percentages were averaged. The resulting average phonetograms for the singers and the nonsingers are shown in Fig. 6. The differences are surprisingly small. 11 0 I I I I I I I I I I I 110 I 110 ?? 0 I I I I I I 1 I I I ------103 100 - 100 100 dX-x /x-xH - - - - -m m - 90 o 90 - 90 3 9C - - - /x-x- /x - E - - m E m -u u m /<* - - 80 0t- ao er, __ta-a -80 G-a - - at- x -=l- - _1 -J /x\x.x i zx:x-x-x - - n 70 - ?o a 70 v, "7 70 ; rM/x\,/~ - yX - - /x.x -

- 60 - 60 65 - 60 - -

I I I I I I I I I I I I I I I I I I 50 I I I I 50 50 . 50 i2 5.4 C E3 GA C EL G A C €5 E2 GA C €3 GA C E4 G A C €5 82,5 98 ",'o 131 165 196 220 262 330 392 4'+0 524 b60 82,5 98 110 131 165 196 220 262 330 392 440 524 660 PHOKATION FREQUENCY PHONATION FREQUENCY Fig. 5. Examples of phonetograms for (left) singer subjects RO (above) and SC (below) and (right) tor nonsinger subjects KR (above) and TO (below). N, L, LL, and M refer to L values for the normal, loud, very loud, and masked conditions, respectively. eq STL-QPSR 1/1987

For canparison we also show in this same figure the average phone- tcgram for normal voices with no or only moderate experience of singing, that was previously published by Coleman & al. (1977). There is a fair agreement with regard to the upper contour, referring to loud maximum phonation; however, our subjects did not reach as high values as in the previous investigation. There is a great difference in the lower contour (for soft phonation), which is 10 to 20 dB lower in the Coleman & al. study. In these phonetograms are also included the trajectories showing how the subjects changed their mean relative fundamental frequency as they changed loudness in the normal, loud, loudest, and noise conditions.

Table I. Results of linear regression for each subject's relationship between mean relative fundamental frequency in semitones (st) and La based on reading in normal, loud and loudest conditions and with masking noise including both trial I and trial 11.

Subject Slope Intcpt Corr. (st/dB) (st) Singers : JEJ STEF ROB SCH SOL JOH ALF ROL Singers BILL mean 0.425 0.947

Nonsingers: JAN MAWTIN AND. A AND. F ERIK m. GODDY TONI Nonsingers MICKE mean 0.411 0.963 FUNDAMENTAL FREQUENCY (% of total range) Fig. 6. Averaged phonetograms for nine singers and nine nonsingers (solid and dashed curves). The bars represent +/- one standard deviation. The fundamental frequency range has been normalized. The crosses show comparable data reported by Coleman & al. (1977). The trajectories show averages observed under conditions of normal (N), loud (L), loudest possible (LL), and masked (M) reading. STL-QPSR 1/1987

Before commenting upon these trajectories, it is necessary to digress for a moment and recall the relationship between the two loudness measures involved. The L is a time average of SPL: eq

where T is the intergration time PA(t) is the SPL as measured with A-weighting Po is the reference pressure (=20 uPa)

Therefore, the L an3 the SPL be equal only under coditions eq will of sustained sounds. In speech, pauses occur and the SPL varies conti- nuously. Therefore, the L will always be lower than the highest SPL in eq the speech sample analyzed. Thus, if the maximum SPL occurs only during 1/10 of the total reading time, the Leq will be 10 dB lower than this maximum SPL, and if the maximum SPL occurs half of the reading time, the difference will be 3 dB. For these reasons, the Lq values slmuld be expected to be rather low as compared with the phonetogram top SPL values, which apply to sustained phonation of the vowel /q.However, the fundamental frequency value assigned to an L--value is an average representing both lower and higher fundamental frequencies. In averaging sound level, high sound levels are more influential than low sound levels. Moreover, pbnation tends to be louder at higher than at lower fundamental frequencies. This has the effect of raising L -values for eq speech with a varying fundamental frequency, or, in other words, it causes the points on the trajectories in the figure to approach the upper phonetogram contour. The subjects' mean fundamental frequency in normal speech lies at about 15 to 20% which corresponds to about 5 semitones above the lowest pitch. For loudest possible speech, it is raised to 40 or 50%, or about one octave above the lowest pitch. The equivalent sound level in normal speech is close to the minimum SPL and approaches maximum SPL for loudest possible speech. The singers are seen to phonate at a higher sound level and at higher mean relative fundamental frequency througbut. They also seem to place their mean fundamental frequency relatively higher than the nonsingers. With respect to the trajectories, the singers are seen to phonate at a higher L and at higher mean relative fundamental frequency eq throughout, as was previously shown in Figs. 2 and 3. Moreover, the nonsingers seem to place their mean fundamental frequency lower in their range than the singers. The loudest codition yielded a point close to the top contour for the singers. This may depend on an articulatory factor: many singers applied a legato style of speaking in loud reading, apparently shortening unvoiced segments. In both the singers and the STL-QPSR 1/1987 - 51 -

nonsingers, the data points pertaining to the noise codition (M) fall close to the line joining the pints for loudest and loud reading. As can be observed in Fig. 6, the voice changes between normal reading and reading in noise are not the same for the singers and the nonsingers. Moreover, these data were comparable with the data collected from the group of male and female dysphonic patients. An interesting parameter therefore seems to be how much the fundamental frequency increased for a given rise in L above the values for normal reading, or, in other words, the ratio between the increase in mean relative fundamental frequency and the increase in Leq' This ratio, henceforth AF~/AL~,expressed in st/d~,is shDm for the various subject groups in Fig. 7. The difference between the groups with regard to A F0/6 L was tested for significance using a Kruskal-Wallis nonparametric one-way analysis of variance. As this test showed a significant effect of the groups (p<0.05), the differences were subsequently tested for significance using a two-tailed Mann-Whitney test. t, 2

-Z 2 W I/) -=l W 1 (Y NORMAL NORMAL INCREASE IN Leq (dB)

Fig. 7. Mean change for nine male singers (S), nine male nonsingers (NS), ten male dysphonic patients (MP) and ten female dysphonic patients (FP) in average fundamental frequency as function of change in L observed when eq' reading in noise as compared with normal reading. STL-QPSR 111987 - 52 -

The results shown in Table I1 indicate that the singers differed significantly (p<0.05) from the nonsingers and the patients, while there was no significant difference between the two last-mentioned groups. Thus, when the singers were reading in noise, they raised mean pitch more and Lq less abwe the normal reading level than did the nonsingers and the patients. With respect to the loudest condition there was no significant difference in this ratio between the singers and the nonsingers.

Table 11. Analysis of variance of the effect of the groups on the increases in mean relative fundamental frequency relative to the increase in L (~FO/AL~:in st/d~)when reading in masking noise as compared eq with normal readmng. t Group N Min Median Max ( st/dB) ( st/d~) ( st/dB)

Singers 9 0.3 0.540 1.250 Nonsingers 9 0.2 0.430 0.640 Dysphonic patients (male) 10 0.15 0.400 0.480 Dysphonic patients (female) 10 0.18 0.320 0.460

Discussion In a study of mean fundamental frequency in speech during noise Kitzing (1979) compared two groups of subjects: teachers of nursery school who had basically untrained voices and voice therapists who had had considerable voice training. It turned out that the trained subjects raised their mean fundamental frequency significantly less than did the untrained subjects. Our finding was that the singers raised their mean fundamental frequency more than did the nonsingers. The difference in these results may be explained by the difference in the voice education of singers and voice therapists; in singing as opposed to normal speech one must be able to change fundamental frequency within a wide range without straining the voice. We found that, when the singers were reading in noise, they raised mean pitch more and L less above the normal reading level than did the eq nonsingers and the patients. One explanation of this would be that the singers possessed a wider phonation frequency range than the nonsingers. Wwever, there was no difference between the groups, neither with respect to L nor to phonation frequency range, as illustrated by the eq ' similarity between the upper and lower countours shown in Fig. 6. Thus, the difference between the groups with regard to AFO/AL~~cannot be explained by a difference in phonation frequency range. STL-QPSR 1/1987 - 53 -

Another explanation may seem more plausible. It is likely that, as compared with nonsingers, singers can use a wider pitch range withDut stressing the voice. They may take advantage of this wider easily avail- able range when reading in noise. In the experiment, two different types of noises were used; the noise used for the healthy subjects was LP filtered (cf., Fig. 1) while the noise used for the patients was white. Also, the SPL differed being 100 dB in the case of the filtered noise and 70 dB in the case of the white noise; however, because of the spectrum dissimilarities, this level difference does not imply any great difference in the loudness perceived. Above 500 Hz, approximately, a vowel spectrum typically falls off at a rate of 6 d~/octave,so the spectrum level at high frequencies is low. The filtered noise seems preferable to use as there is no need to bother subjects with masking noise energy in frequency regions that are practically void of sound energy. However, the choice does not seem critical to the effects induced. The patients and the untrained subjects reacted in similar ways, although the noise was different . It is interesting that there was no clear difference between the nonsingers and the patients regarding the increases in SPL and in mean fundamental frequency observed during reading in noise. Thus, our method does not seem useful for discriminating normal and pathological voices. However, as shown by Kitzing (1979), differences appear after an ex- tended period of reading in noise. Our lower phonetogram contour was similar to that reported by Seidner, Kreuger & Wernecke (1985) but considerably higher than that observed by Coleman & al. (1977). Cur higher contour is similar to that found by Coleman & al. but much higher than than found by Seidner & al. for nonsingers. The reason for these diverging results is probably differences in the procedures and in the vowel used. An interesting aspect of the relationship between shifts in L and fundamental fre- eq quency is the fact that loudness level is regulated by subglottal pressure in the first place: according to Bouhuys, Mead, Proctor & Stevens (1968), a doubling of subglottal pressure tends to result in an increase of SPL of 9 dB, on the average. rnwever, a raised subglottal pressure also yields a rise in fundamental frequency: according to Baer (1979) the voice fundamental frequency increases by about 4 Hz per cm H20. In normal speech, subglottal pressure is typically in the vicinity of 5 cm H20. A loudness increase of 9 or 18 dB, can then be estimated to result from an increase of subglottal pressure by a factor of 2 or 4, respectively, i.e., from 5 to 10 or to 20 cm H20. Such increases would induce fundamental frequency rises of about 20 or 60 Hz; counted from 100 Hz, this increase corresponds to about 3 ad 8 st. We observed that the mean fundamental frequency rose by around 0.4 st/d~. For 9 dB this yields a rise of 3.6 st, ard for an 18 dB increase STL-QPSR 1/1987

a rise of 7.2 st. Thus, the observed rises in mean fundamental frequency are of the same order of magnitude as tbse expected from the postulated increases in subglottal pressure. In other words, the changes in mean fundamental frequency could very well be the result of the sutglottal pressure changes. The high correlation between mean fundamental frequency an3 L in eq reading probably reflects a habit in voice use under the experimental conditions used. In other words, it is certainly possible to combine loud reading with a low voice pitch or soft reading with a high voice pitch, if needed. In clinical work reading in noise and phonetograms are both methods that are frequently used. In the present investigation both these methods have been applied. Still, it is hard to compare their clinical value, as no phonetograms were made for the patients. Also, we lack independent data on voice function. QI the other hand, it may be worth- while to analyze in relation to the phonetogram L and mean fundamental frequency for voice patients speaking at different vocal efforts. It is possible that it is more appropriate to speak about an optimal pathway in the phonetogram rather than an optimal voice pitch.

Conclusions Singers phonated at higher mean L and at higher mean relative eq fundamental frequency than nonsingers when reading with normal and raised loudness and during noise. When reading in noise, the singers raised their mean fundamental frequency more and the mean L less above eq the values observed for normal reading than the nansingers. M difference was found between nonsingers and dysphonic patients in this respect. The Lq and mean fundamental frequency were highly correlated; the subglottal pressure increases that can be postulated from the increases in L are sufficient to explain the rises in mean fundamental frequency eq as passively induced by the pressure rises. The correlation probably reflects a habit in the use of the voice. Mean phonetograms for singers and nonsingers differed consistently only in that the nonsingers had a slightly lower level at the top frequencies of the upper contour. The phonetogram seems rather sensi- tive to the procedure used for recording it, as subjects would easily avoid extreme degrees of loudness.

The subjects are gratefully acknowledged for their kind coopera- tion, the Department of F'honiatrics, Malmo General Hospital for provid- ing access to the patient groups, as well as to recording and computer STL-QPSR 1/1987 - 55 -

facilities, Statistician J.A. Nilsson for assisting with statistical processing of data, Ing. L. akerlund for technical assistance in recording the patients ad analysis, and Professor N.G. Tbremalm for valuable support in many different ways.

References Baer, T. (1979): "Reflex activation of laryngeal msucles by sudden induced subglottal pressure changes", J.Acoust.Soc.Arn. -65, pp. 1271- 1275.

Bouhuys, A., Mead, J., Proctor, D.F. & Stevens, K.N. (1968): "Pressure- flow events during singing", Ann. of the New York Acad. of Sci., -155, Art. 1, pp. 165-176.

Coleman, R., Mabis, J. & Hinson, J. (1977): "Fundamental frequency - sound pressure level profiles of adult male and female voices", J. Speech and Hear .Res. -20, pp. 197-204. Fant, G. (1979): "Glottal source and excitation analysis", STL-QPSR 1/1979, pp 85-107.

Fant, G., Liljencrants, J. & Lin, Q. (1985): "A four-parameter model of glottal flow", S'I'L-QPSR 4/1985, pp. 1-13.

Gauffin, J. & Sundberg, J. (1980): "Data on the glottal voice source behavior in vowel production", SIGQPSR 2-31 1980, pp. 61-70.

Kitzing, P. ( 1979): "Glottografisk frekvensindikering (Glottographic Frequency Indicator)", bt. diss., Lund University.

Lane, H. & Tranel, B. (1971): "The kmbrad sign and the role of hearing in speech", J. Speech and Hear.Res. -14, pp. 677-709.

Perkins, W.H., Ternstrom, S., Sundberg, J. & Gramming, P. (1987): "Effects of high-frequency feedback and noise on phonation as analyzed by long-term-average spectra", STGQPSR 1/1987, pp. 57-80.

Schutte, H.K. & Seidner, W. (1983): "Recommendation by the Union of European Phoniatricians (UEP): Standardizing voice area measurement/ phonetography", Fol. Phoniat. -5 pp. 286-288.

Seidner, W., Krueger, H. & Wernecke, K-D. (1985): "Numerische Aus- wertung spektraler Stimrnfelder", Sprache-Stimme-Geh6r 9, pp. 10-13.